Field of the Invention
The present invention relates to an improved optical fibre monitoring device.
An optical fibre monitoring system uses two essential components, namely a light source and a photoreceiver. In the case of a device of the "direct barrier" type, the photoreceiver faces the source. In the case of a so-called "reflex" barrier, a catadioptric reflector is also positioned facing the source and the photoreceiver is positioned alongside the latter. This arrangement can even be used without a reflector, if it is the reflectivity of the object to be detected which is used and then a so-called "proximity" optical system is obtained.
The advent of optical fibres made it possible to improve such devices. Thus, optical fibres have interesting qualities, such as insensitivity to electromagnetic interference and inviolability of the information carried by them. In the case of silica fibres, there are additional advantages such as the limited attenuation in the near infrared, the ease of fitting (due to the small diameter and great flexibility), the good thermal behaviour and the good resistance to chemical action and radiation. Moreover, optical fibres have been recently used not only in telecommunications, but also in the construction of monitoring devices. They have varied applications, such as the detection of intrusions, the detection and counting of objects, security, etc.
The following table gives an idea of the scope obtained with existing commercial devices, as a function of the core diameter of the fibre used and as a function of whether or not end optics are available in the three aforementioned barrier types.
______________________________________ Fibres with 200 .mu.m fibres 1 to 2 mm fibres end optics, System without end without end dia. 30 to type optics optics 40 mm ______________________________________ Direct 3 to 10 cm 5 to 50 cm 5 to 50 m barrier Reflex 1 to 20 cm 2 to 100 cm 1 to 50 m barrier Proximity &lt;2 cm &lt;10 cm &lt;0.5 m ______________________________________
200 .mu.m fibres generally have a silica core and a plastic sheath, or a silica core and a silica sheath, in a structure similar to that of the multimode fibres used for telecommunications purposes. The optical attenuation introduced by them remains negligible for lengths below about 100 meters.
1 to 2 mm fibres are either plastic fibres (being the least expensive), or bundled glass fibres. The attenuation introduced by them can reach several dB/m, which leads to a significant decrease in the effective scope of the associated system when using non-negligible fibre lengths (several meters).
FIG. 1 diagrammatically shows the structure of an optical fibre monitoring device. Such a device comprises a lightemitting diode 10 coupled to an optical transmission fibre 20, a photoreceiver 14 coupled to an optical reception fibre 22 and a control system 15. This system comprises a module 12 for controlling the emission of the light-emitting diode 10, a preamplifier module 16 connected to a photoreceiver 14 and a module 30 for processing the preamplified signal connected to preamplifier 16. System 15 also comprises a block 36 for supplying the different modules, indicator lights 32 and outputs 34 (analog and/or logic).
Volume 21 between the free ends of the transmission and reception fibres 20, 22 respectively corresponds to the monitoring zone.